The present invention relates to a carrier of a developer, and more particularly to such a carrier to be used in a two-component type developer for developing an electrostatic latent image as well as a means for determining the dynamic permittivity of the carrier.
Conventionally, there has been widely used an image forming apparatus such as an electrophotographic copying apparatus using a Carlson process. The Carlson process generally includes, as basic (indispensable) steps, the steps of:
charging a photoreceptor by corona discharge or the like: PA1 exposing the uniformly charged photoreceptor to form an electrostatic latent image corresponding to the image of an original; PA1 developing the electrostatic latent image thus formed into a toner image by a developer; PA1 transferring the toner image thus formed on the photoreceptor to a base material such as copying paper; PA1 fixing the toner thus transferred to the base material; and PA1 cleaning the toner remaining on the photoreceptor. PA1 C denotes electrostatic capacity, PA1 .epsilon.o denotes permittivity in vacuum, PA1 S denotes surface area of detector, and the electrostatic capacity C is represented by the following formula: EQU C=I/(.omega..times.V)
A two-component type developer containing toner and a carrier is generally used as a developer in the developing step of the Carlson process.
The carrier used is divided into two groups, i.e., a non-coated carrier such as iron oxide powder, ferrite powder or the like, and a carrier coated with, for example, polyester, a fluorine polymer, or the like. The coated carrier is generally excellent in image characteristics and triboelectrification with the toner. Further, the electric resistance or the like thereof can be easily controlled.
The carrier exerts a great influence in the characteristics of triboelectrification with the toner, as well as the image characteristics such as image reproducibility or the like of half-tone portions and solid portions of an original.
In view of the foregoing, there has been proposed a carrier coated with a high resistance substance containing a substance having a high permittivity (Japanese Publication for Unexamined Patent Application No. 19157/1985).
The carrier disclosed in the publication above-mentioned is advantageously excellent in image reproducibilities of the half-tone portions and the solid portions of an original, since the high-resistance substance such as acrylic resin or the like contains a substance having a high permittivity such as barium titanate or the like.
In the carrier above-mentioned, the coating layer for coating the carrier core material made of iron oxide, ferrite or the like, is made of a high-resistance substance containing a substance having a high permittivity. This results in insufficient mechanical strength of the coating layer. It requires the use of a great amount of resin to enhance the mechanical strength. Further, the image reproducibility of the solid portions is still insufficient and consequently a fog is apt to occur. In particular, when a reproduced image is formed from an original having a stripe pattern containing, for example, solid portions arranged at spatial intervals of about 1 to 3 mm, the rear ends of the solid portions cannot be developed in a faithful manner, producing blurs on the reproduced image.
Thus, there is the problem that the image reproducibility of the solid portions is not sufficient.